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    <div>
        求出1-1/2+1/3-1/4…..1/100的和 var i=1;
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        封装一个函数，他的作用，随意输入一个数，求出从1到以这个为分母的数错位加减的和
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    <script>
        // var res = 0 ;
        // for (var i = 1 ; i <= 100 ; i ++ ) {
        //     if ( i % 2 == 1 ) {
        //         res = res + ( 1 / i ) ;  
        //     } else if ( i % 2 == 0 ) {
        //         res = res - ( 1 / i );
        //     }
        // }
        // console.log(res);
        function mathFunc (a) {
            var res = 0 ;
            for ( var i = 1 ; i <= a ; i ++ ) {
                if ( i % 2 == 0 ) {
                    res = res - ( 1 / i ) ;
                } else if ( i % 2 == 1 ) {
                    res = res + ( 1 / i ) ;
                }
            }
            alert(res);
        }
        var num = parseInt(prompt("请输入一个数"));
    </script>
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